Factors determining seed persistence of Chondrilla juncea L. (skeleton weed) in southern Western Australia

Except when sown into non‐wetting soils, buried seeds (achenes) of Chondrilla juncea germinated readily following summer rainfall events of less than 10 mm. Seeds lying on the surface were much less likely to germinate in response to small rainfall events, but were prone to predation by seed‐harvesting ants. Although seedlings were capable of resuming growth following temporary dehydration at the earliest stages of emergence from the pericarp, very few of these were able to establish. Establishment from summer germination was virtually nil. The capacity to germinate at higher temperatures was increased following storage for seeds which had been produced in February and March. However, this was considered to be only a weak expression of a release from relative dormancy, compared to the marked changes in temperature response which occur during the after‐ripening of many winter annuals. Differences in the germination behaviour and persistence of seeds of the narrow‐leafed and broad‐leafed forms of C. juncea , when evident, were usually slight. On the basis of an analysis of climatic records for 13 wheatbelt sites, obtained over the period for which C. juncea has been known to occur in Western Australia, it is argued that more than two germinating events could be expected to occur, on average, during the summer months. The relative absence of mechanisms which might confer protection from the effects of unseasonal rainfall, in conjunction with evidence for high levels of seed predation, point to considerable seed losses. Thus the abundance of the species elsewhere in Australia appears to have resulted from its capacity for vegetative regeneration.